Mitral Valve Echocardiography from the Apical Four Chamber View
©Walter Rasmussen, R.D.C.S.
Apical View of the Mitral Valve
The apical 4-chamber view is one of the best for imaging the mitral valve and with most patients, a bright, focused image it is not difficult to obtain. After obtaining the general four-chamber view, use the zoom function to enlarge the mitral valve and annulus. Respiratory maneuvers with suspended inhalation or exhalation at the point where the valve appears bright and focused, can greatly enhance the diagnostic value of the image. If there is calcification of the mitral annulus or valve leaflets, the high reflectivity of these structures will make the image of the valve appear much brighter than the surrounding structures. Making use of the sliding TGC control tabs corresponding to the depth of the valve in order to reduce the brightness will reveal more detail of the valve structure.
Using higher frequencies (High Resolution adjustments) will produce significantly better images of the mitral valve when there is sufficient beam penetration. The section of this guide that describes how to compensate for the increased noise that is often encountered with the use of higher frequencies will greatly aid in achieving this goal. Routine imaging of the mitral valve with low resolution adjustments will not provide the best quality examination to the patient and physicians who rely upon sonographers to provide good service.
Color Doppler of Blood Flow across the Mitral Valve:
Below: Mitral Valve Color Doppler, Diastole and Systole
Ideally, color Doppler should convey the movement of the blood volume both in diastole and systole, therefore, adjusting the color Doppler scale, reducing the image sector width and using a color box of minimum width and length in order to keep the frame rate high (at least 17 frames per second), helps to accomplish this goal. If the color just flashes, the frame rate is too slow and should be adjusted upward using the above methods. Poor return of the ultrasound can also result in faint, flashing color Doppler when the gain is set too low. Make certain that the wall filter is not set too high by lowering its setting until just above the point of flashing artifact with will interfere with interpretation of the information. Color Doppler gain should be adjusted to the point at which background color speckling is seen, and then backing-off until speckling is minimal in order to avoid under-estimation of jet length and area.
The color Doppler software does not alert the sonographer when poor penetration of the ultrasound is occurring. The only way to assure that the color Doppler display is producing an accurate representation of blood flow is to look at clarity of the two dimensional image. In fact, even if there is a seemingly good color Doppler image of mitral regurgitation or any other blood flow, a background 2-D image of poor quality, the accuracy of the Color Doppler image should be suspect. Make sure that the color Doppler frequency is not set higher than the recommended 1.9-2.2 MHz level. On some machines, color Doppler frequency control is automated and cannot be manually adjusted.
Directing the patient in using breath control techniques and frequent adjustments to gain and wall filters are every bit as necessary in color Doppler as they are with two-dimensional imaging. A review of one or more of the patient’s previous echocardiograms will aid in producing a study that is consistent with the patient’s history.
Mitral regurgitation jets can originate from any part of the valve commissures, and so it is important to pan the sector in all directions across the mitral apparatus in every view. This panning technique should be used on all of the valves and in all of the views. Whenever a jet is narrow and directed toward a wall, effort should be made to follow it by changing the transducer angulation and axis so that its full length can be observed.
Once the presence of regurgitation and the jet area is documented, color Doppler of mitral inflow is used for accurate positioning of the CW cursor and PW sample volume for assessment of systolic and diastolic flow. If there is mitral stenosis or global left ventricular hypokinesis, it is important to show the full extent of the inflow jet for characterization of filling pressure.
Color M-Mode is a color Doppler technique that helps to qualify the degree of left ventricular dysfunction. The technique consists of placing the color Doppler box at the mitral inflow and lengthening it to include the complete area of mitral inflow. The cursor is placed across the mitral valve and with the activation of M-Mode, the color Doppler map is superimposed on top of it, demonstrating a high speed record of the inflow pattern. The color Doppler mitral inflow diastolic slope can then be measured.
A jet of aortic insufficiency might interfere with the recording of diastolic blood flow across the mitral valve with CW and PW Doppler. If possible, try to position the cursor outside of the main portion of the AI jet by repositioning the transducer a bit more laterally. PW Doppler usually is able to filter out much of the interference of aortic insufficiency and often renders a good mitral inflow velocity profile when minor adjustments are made.
Spectral Doppler of the Mitral Valve:
Duplex Doppler allows for precise guidance of CW Doppler Cursor
While color Doppler is excellent for locating blood flow and determining its direction and a rough estimation of its speed, spectral Doppler can determine its exact velocity. Spectral Doppler of diastolic blood flow across the mitral valve is used for the assessment mitral stenosis and of left ventricular filling characteristics. Retrograde blood flow across the mitral valve aids in the determination of the severity of mitral regurgitation and pressure relationships between the left ventricle and left atrium.
Once mitral regurgitation is located, place the CW cursor over the origin of the jet where it emerges from the valve leaflets during systole, (often referred to as the vena contracta), and initiate the CW Spectral Doppler display. Many machines require that a crosshair be placed at the level of the vena-contracta for optimal focusing of the ultrasound beam and this feature can make a significant difference to the quality of the spectral Doppler recording.
Since the left ventricle typically generates between 100 and 150mmHg of pressure during systole, the CW Doppler scale and baseline should be adjusted so that the spectral recording of the MR jet fits well within the chart below the baseline, and so that diastolic flow above the baseline, is not cut off at the top of the chart. Even if there is little or no mitral regurgitation present, it is important to demonstrate it with spectral Doppler.
The CW Doppler gain knob is used to enhance or diminish the strength of the recording. When mitral regurgitation is being investigated, it is necessary to increase the gain specifically for the best recording of the mitral regurgitation velocity profile without regard for the intensity of diastolic flow above the baseline because the MR signal is usually much weaker than the diastolic signal.
Dynamic Range, Compression, and various other contrast-enhancing adjustments on machines of different manufacture, will help to define the border of the spectral Doppler recording. It is also important to increase the gain just enough to show a small amount of noise, seen as very light grey or speckling, in the background of the recording in order to assure that the full velocity profile will be displayed. The goal is to get a complete, uninterrupted recording of the jet throughout systole however, unless the degree of mitral regurgitation is at least moderate, it is unlikely that the velocity profile will be complete. It is not necessary to spend an inordinate amount of time seeking a perfect velocity profile for mild mitral regurgitation after a reasonable amount of un-successful effort.
CW Doppler is also used to assess peak diastolic flow across the mitral valve when mitral stenosis or a prosthetic valve is present. When assessing diastolic mitral blood flow with CW Doppler, lower the scale and move the baseline downward so that a clear and measurable velocity profile can be obtained above the baseline. Adjust the gain and dynamic range until the sharpest possible border to the velocity profile is obtained. Of particular importance is the E-F slope which should be as sharply defined as possible for accurate measurement of the pressure half-time.
Pulsed-Wave Doppler of the Mitral Valve:
PW Doppler of the Mitral Valve from the apical window
Placement of the PW cursor with the aid of color Doppler is of utmost importance because early and late mitral inflow recordings can be adversely affected if the sample volume is placed too close to the left ventricular outflow tract or too far from the mitral valve leaflet tips.
The PW Doppler exam of the mitral valve follows the CW Doppler exam. Using Color Doppler as a guide, position the PW sample volume just beyond the valve leaflet tips (on the ventricular side of the valve), within the brightest red area of the Color Doppler map. Observe the diastolic flow pattern on the spectral Doppler strip chart by moving the baseline downward and adjusting the scale so that profile is large enough to view and measure accurately. Explore the area with the goal of documenting the highest velocities for the E and A points. A sharper velocity profile of the mitral valve can usually be obtained with PW Doppler rather than CW Doppler because it limits the sampled area whereas; CW Doppler mixes all the blood velocities encountered by the ultrasound beam.
PW Doppler velocities are limited to a considerably lower scale range than CW Doppler due to the Nyquist limit and occasionally, the diastolic velocity will exceed the capabilities of the pulsed technique due to valve stenosis, prosthesis or increased distance. In these instances, the baseline will have to be moved to the very bottom of the chart, totally excluding the blood flow below the baseline. If the velocity profile still does not fit on the graph, the scale is repeatedly elevated in an effort to fit the diastolic flow pattern and depending on the distance, high pulse repetition frequency Doppler may be activated automatically by the ultrasound system, incorporating more than one sample volume in to the velocity profile. The sonographer should be aware of the PRF status at all times and consider how it may affect interpretation of the Doppler information. If mitral stenosis or a prosthetic valve is present, diastolic blood flow should be recorded with both CW Doppler and PW Doppler at a similar scale and the velocities compared. The tracing with the highest diastolic velocity is preferred for making measurements and calculations.
Another way of increasing the Nyquist limit in order to record higher velocities without using high PRF PW Doppler on some ultrasound machines is to reduce the size of the sample volume.